With the rapid development of microelectronics, the technology of central processing units (CPUs) gradually matures. While the processing efficiency and speed of CPUs are improving, the increase of their operating clocks leads to the generation of more heat during operation; therefore, the demand for heat dissipation is growing. To deal with such a demand, a novel fan was developed using the pulse-width modulation (PWM) technique. The PWM fan is used in conjunction with a computer which when in operation generates a PWM voltage signal according to the temperature of its CPU. A PWM voltage signal is composed of many digital pulses and provides control in a way similar to analog signals. The proportion of time of the pulses in a PWM voltage signal is referred to as the “duty cycle”. For example, a duty cycle of 10% means that the computer drives the PWM fan into operation in 10% of the time and leaves the PWM fan idle in the remaining 90% of the time. By varying the duty cycle, the computer can effectively adjust the rotating speed of the fan to ensure proper operation of the CPU.
Compared with the conventional fans, a PWM fan features higher heat dissipation efficiency and precision. However, as a PWM fan must be equipped with a corresponding control chip in order to receive PWM voltage signals, the production costs of PWM fans are far higher than those of the conventional fans, which are typically controlled by the level of voltage. In order for a conventional fan to receive PWM voltage signals and work as a PWM fan, a control circuit was developed as shown in FIG. 1. The control circuit 1 in FIG. 1 is composed of a filter circuit 11, an operational amplifier 12, a switch circuit 13, and a feedback circuit 14. The filter circuit 11 is configured to receive the PWM voltage signal transmitted from a processing unit and convert the PWM voltage signal into a direct-current (DC) voltage signal, wherein the magnitude of the DC voltage signal is in direct proportion to the duty cycle of the PWM voltage signal. The operational amplifier 12 is configured to amplify the DC voltage signal so as to control the transistor Q in the switch circuit 13 and instruct the transistor Q to output a driving voltage signal to a fan 10.
The control circuit 1 nevertheless has several drawbacks in use, and the most important reason is this: the driving voltage of a conventional fan is approximately between 4 and 13 V, but the PWM voltage signal, whose duty cycle ranges from 0% to 100%, is converted by the control circuit 1 to 0˜13 V. Since a voltage of 0˜4 V cannot drive the fan 10 into operation, a PWM voltage signal whose duty cycle is 0˜15% does not work for the fan 10 (assuming a PWM voltage signal whose duty cycle is 15% is converted to 4 V). That is to say, a “non-correspondence in signal conversion” arises.
Another important issue with the control circuit 1 is that the operation of the fan 10 cannot be effectively stopped, and this is due to the fact that the control circuit 1 controls the value of the driving voltage output from the switch circuit 13 to the fan 10 based on the output voltage of the operational amplifier 12. Referring to FIG. 1, the output voltage of the operational amplifier 12 is supplied to the control end of the transistor Q so that the voltage difference between the other two ends of the transistor Q can be adjusted according to the output voltage. In other words, in order for the control circuit 1 to stop the operation of the fan 10, the output voltage of the operational amplifier 12 must be equal to the supply voltage of the switch circuit 13 (e.g., 12 V). However, since the supply voltage for driving the operational amplifier 12 and the switch circuit 13 is in most cases provided by the same supply unit Vcc, the output voltage of the operational amplifier 12 can never be as high as the supply voltage, simply considering the loss in the operational amplifier 12; consequently, the operation of the fan 10 cannot be effectively stopped. The control circuit 1, therefore, exhibits “poor control”.
In light of the above, the inventor of the present invention wondered if it is possible to design a novel circuit structure which not only can solve the lowest driving voltage problem of the conventional fans, but also can stop the operation of a fan precisely at any moment according to the state of a computer. The issue to be addressed by the present invention, hence, is to overcome the aforesaid problems by providing an improved and easy-to-manufacture circuit structure.